It is known to use wideband receivers for direction finding. Typically a system comprises a circular array of wide angled antennas, each of which is coupled to a respective wideband receiver. At the detection of a signal within the frequency range of the wideband receiver, an output proportional to the amplitude of the signal as received is produced. The respective amplitude signals are supplied to a bearing processor which determines the bearing of the source of signals. If it is desired to measure characteristics of the received signal, such as its frequency, then for example an instantaneous frequency measuring device is coupled to either an omni-directional antenna or a combination of the bearing array antennas so arranged as to give omni-coverage. The use of wideband receivers for direction finding has a number of limitations. In particular, they are not suitable for measuring characteristics (or emitter parameters) in increasingly dense scenarios in which simultaneously occurring signals have different characteristics and relatively short durations.